Thin film devices typically comprise a multiplicity of metal wiring layers separated by insulating, or dielectric, material layers to provide electrical isolation. If the dielectric used to electrically isolate the wiring layers of a thin film circuit is a perfect insulator, normal handling of the chips cut from fabricated wafers can lead to the accumulation of electrostatic charge and the formation of very large potential differences between the thin-film layers of the device. An electrostatic discharge (ESD) occurs if the potential difference becomes too large, which can damage or destroy the circuit. In order to minimize device failure owing to electrostatic discharge, special ESD-safe workstations and tools are required, and all personnel handling the circuits must wear grounded wrist straps and heel straps. Anti-static lotions and smocks also are commonly used when handling sensitive circuits.
Thus, it would be desirable to provide a device which minimizes the potential difference between adjacent metal wiring layers during normal handling, but which also provides the desired isolation between such layers during operation, when such isolation is functionally required. Specifically, for superconducting devices it would be desirable to provide a transition layer which provides conductance between adjacent metal layers during handling at room temperature, when the potential for ESD is greatest, but which acts as an insulator between the metal layers during operation, at temperatures below about 100 Kelvins (K).
Devices incorporating layers of material capable of undergoing an insulator to metal transition are known. For example, in U.S. Pat. No. 6,365,913 to Misewich et al., a Field effect transistor semiconductor switch is disclosed in which the channel is made from materials having an electrical conductivity which can undergo an insulator-metal transition (i.e., Mott transition) upon application of an electric field. Likewise, U.S. Pat. No. 5,721,197 to Downar et al. discloses a microelectronic thin-film device having a thin superconducting layer in contact with a thin quasi-insulating conversion layer. The critical current of the superconducting layer is controlled by application of a voltage to the conversion layer by means of a gate electrode, causing the conversion layer to undergo an insulator-metal transition. Both references require the application of a voltage or electric field in order to change the conductivity of the transition material.
Thus, there remains a need for a device that allows for conductance between metal layers during normal handling at room temperatures, thus minimizing the potential for destructive ESD, but which provides the desired electrical isolation between metal layers during normal operation at the low operating temperatures associated with superconducting devices.